1. Field of the Invention
The present invention relates to a display apparatus, a driving apparatus for light-emitting devices, and an image forming apparatus, and, more particularly, to a display apparatus including organic electroluminescent (EL) devices and driving circuits for the organic electroluminescent devices.
2. Description of the Related Art
Light-emitting devices that utilize the electroluminescence of an organic compound are arranged in a matrix and used in a display apparatus. In an active matrix display apparatus, a driving circuit is provided for each pixel, and supplies current according to data voltage to each organic EL device. At this time, because the threshold voltage of a driving transistor included in each driving circuit is not the same, there is a problem in that the current supplied to the organic EL device varies between the pixels.
In International Publication No. WO 98/48403, a driving circuit that generates current independent of the threshold voltage of a driving transistor is disclosed in FIG. 3. Prior to writing of data voltage, a current path between the driving transistor and an organic EL device is blocked, and a gate and a drain are short-circuited to each other. In doing so, the drain current of the driving transistor discharges a gate-source capacitor, thereby decreasing gate-source voltage. When the gate-source voltage becomes equal to the threshold voltage of the driving transistor, the drain current becomes zero. Therefore, the capacitor holds the threshold voltage. Such an operation for setting the gate-source voltage to the threshold voltage of a transistor using current flowing through the transistor is called an auto-zero operation.
In order to write data voltage to a driving circuit in which the threshold voltage is held by a gate-source capacitor of a driving transistor as a result of the auto-zero operation, a change in the voltage of a data line is transmitted to the gate of the driving transistor through another capacitor connected between the gate and the data line. When the voltage of the data line is changed from reference voltage at the time of the auto-zero operation to the data voltage, voltages at both ends of the gate-source capacitor change from the threshold voltage by a voltage proportionate to the change in the voltage of the data line. The gate-source voltage after the change has a value obtained by adding the change proportionate to the data voltage to the threshold voltage. Therefore, drain current independent of the threshold voltage may be obtained.
In U.S. Patent Application Publication No. 2003/0030603, a driving circuit for a light-emitting device that uses an operational amplifier and that obtains driving voltage independent of a threshold is disclosed.
The luminance signal voltage for the light-emitting device is determined as one input of the operational amplifier, and the voltage of the light-emitting device connected to a source or a drain of a driving transistor is determined as another input of the operational amplifier as a feedback signal. An output of the operational amplifier is connected to a gate of the driving transistor. Because of the operation of the operational amplifier, the voltage of the light-emitting device may become the same as the luminance signal voltage regardless of the threshold voltage of the driving transistor. However, it is extremely difficult to configure a feedback loop using a data line and a feedback line and to cause the operational amplifier to stably perform a feedback operation. Because the data line and the feedback line have relatively large capacitance and resistance components, it takes time to achieve a stable point. In addition, because the data line and the feedback line have inductance, there is a problem in that oscillation is likely to occur.